Is it possible to use the Radeon HD 2400 Card as Primary device and if yes how?

Yes it is. In the UBoot menu there should be an option to switch the primary graphics device, at least there is on the A1. Once this is switched, UBoot will show on the HD 2400, and Amiga OS 4 will automatically use it as the primary graphics card.

At this stage I would not recommend using the Radeon HD 2400 as the primary graphics card because it has no hardware acceleration at all. This makes all graphics rather slow on anything but the lowest resolutions.

Will all the 256MB Videomem usuable with OS4.1?

It should do, although some cards have reduced apertures. For example, the Radeon HD 4350 that I have has 512 MB, but only 256 MB of that is directly visible via PCI to the CPU. I have also seen a few Radeon HD 2400s that have 256 MB, but only 128 MB accessible via PCI.

The problem with Radeon cards was that some of them split the 256 MB up over two PCI "devices" (a primary and secondary device), meaning that the memory could be split in two. All Radeon HD cards that I have seen so far have all the graphics RAM as one device.

I've had a go at trying to get my radeonhd card working with my sam 440ep. But no luck. All i get is a little flashing cursor inside a little square on the top left side on the screen. Nothing from the onboard gfx card. And after some time the screen looses the signal. Did use the installer,but had to rename the monitor driver to only radeon,was radeon m9. Then the installer worked. Also set the pci on uboot menu. No idea if the radeon is working as it should,have no other machine to test it on.

The flashing cursor tells me that UBoot has chosen the other graphics card to be the primary card. UBoot should be displaying on the onboard graphics card. Seeing as it's not, you have a UBoot configuration issue.

The driver switches off the screen when it initializes. It's then up to the OS to open a screen on the card. Did you check the output from the onboard graphics card when this happens?

What do you mean when you say "Also set the pci on uboot menu"?

You could try hooking up your SAM's serial port to another computer to see if UBoot spits out any information.